1. Field of the Invention
The invention generally relates to a sealing device for turbine engines. Specifically, the invention is a segmented intershaft seal having a cross section which is generally either T-shaped or L-shaped. Each seal segment is biased in an outward radial direction so as to contact an inner annular surface along one or both end rings disposed about the seal. Seal segments are further seated onto one or both end rings via centrifugal forces and pressures during operation a turbine engine.
2. Background
Applicant describes an intershaft seal for counter-rotating and co-rotating turbine engines in U.S. patent application Ser. No. 12/210,246 and PCT Application Number PCT/US2009/052328. The intershaft seal is a clearance-type seal applicable between inner and outer shafts of a turbine engine. The seal assembly includes a pair of end rings and a seal element. The end rings are secured to an inner shaft. At least one end ring has a flange, with upper and lower surfaces, disposed along and extending from one side of the ring. The seal element includes a ring, with inner and outer surfaces, and a ring flange. The ring flange is disposed along and extends diametrically outward from the outer surface. The seal element is disposed between the end rings so that the ring flange extends beyond the flanges. The outer radial surface of the ring seats with the lower surface along at least one end ring when rotation of the inner shaft imparts an outward centrifugal force onto the seal ring.
The intershaft seal described above minimizes wear and friction induced heat by contacting the inner shaft but not the outer shaft during normal operating conditions; however, heat and thrust induced misalignments between the shafts do occur from time-to-time causing the seal to briefly contact the outer shaft. When contact between the intershaft seal and outer shaft is properly minimized, the weight of the seal is relatively constant during the operational life of the seal. This feature ensures a relatively constant outward centrifugal force which secures the intershaft seal to end rings along the inner shaft when the inner shaft is rotated.
The diameter of an intershaft seal is often constrained by manufacturing considerations. In one example, the uniformity and consistency of material properties is inversely related to the size of the material stock, examples including a sheet, bar, or block, from which the seal ring is fabricated. This means that larger diameter seal rings are inherently less uniform mechanically and thermally when manufactured from a single, large material stock. In other example, material and manufacturing costs increase with the size of the material stock. Materials costs increase with stock size by virtue of the raw material, processing, and handling costs. Manufacturing costs increase with stock size because of the equipment required to handle and properly machine a seal to proscribed tolerances and the waste or scrap associated with unused portions of the stock material. In yet another example, manufacturability decreases inversely with stock size because of the difficulties associated with holding proscribed tolerances and avoiding warp and other manufacturing induced defects which may or may not be related to imperfections or inconsistencies within the stock material.
One solution is to construct a large diameter intershaft seal from two or more segments which interlock to form a single ring structure whereby each seal segment is manufactured from a separate, smaller, more uniform, and less costly material stock. However, this approach is problematic for the intershaft seal described above because the seal, by virtue of its design and function, is unsupported before centrifugal forces cause it to move radially outward to seat onto flanges along the end rings. The result is a segmented seal which is difficult to properly assemble within a turbine engine and difficult to properly control below the centrifugal forces and pressures which secure the seal ring to the end rings.
Accordingly, what is required is a segmented intershaft seal capable of replicating the performance characteristics of a non-segmented intershaft seal in large diameter applications while avoiding the problems inherent to these applications.